The majority of agricultural pesticide, fertilizer and other agents are applied in liquid sprays. The goal of an optimal spraying system is to deliver and deposit a precise amount of material uniformly and exclusively upon a target area. In a perfect system, only an exact (minimum) amount of material required would be released and all material discharged from the sprayer would ultimately reach the desired target. Industrial spray systems, when operating in controlled environments with well-defined, non-variable targets can often achieve near-optimal performance.
However, other systems such as agricultural sprayers often operate in adverse environments with poorly defined and highly variable target geometries. The operating parameters of an agricultural field sprayer are typically set by the operator at the start of the season and seldom, if ever, modified for changes in the target crop. As the crop morphology changes due to the plant growth or simple variation within a field, the effective application rate of the material (per unit of target crop) varies accordingly. In areas of the field where spray target volume, mass or area is sparse, excessive material may be released and correspondingly, in areas of dense target, poor spray deposition may result in reduced biological efficacy of applied pesticides.
Recently the environmental effects of pesticide use in agriculture have come under much public scrutiny. Concern about possible pollution of the environment has led to tighter restrictions on the use and methods of application and licensing of agricultural pesticides. It has been estimated that at least one billion dollars have been lost annually due to the excessive and inefficient application of agricultural chemicals. One study reported that only 25% of commercial applicators and farm operators applied chemicals within 5% of the proper application rate, and that misapplication ranged from 60% under-application to more than 90% over-application. Thus, the need has become apparent for increasing the accuracy and efficiency of the pesticide application process in order to reduce potential environmental pollution and decrease the overall amount of chemical applied.
Two common ways of expressing application rate in agricultural spraying are active chemical rate and spray rate. ASAE Standard Engineering Practice EP327.1 (1988) defines active chemical rate as "The amount of active ingredient applied per unit treated, expressed in terms of mass per relevant unit treated, i.e. (kg/ha)" and spray rate as "The amount of spray liquid [emitted by an application unit during treatment] expressed in liquid volume per unit treated, i.e. (L/ha)".
Adjustment of application rates for a given agricultural sprayer has historically been accomplished by changing the system pressure (and correspondingly, the flowrate) and/or varying the ground speed. There are numerous drawbacks to these methods of controlling chemical application. First, a constant ground speed is difficult to maintain in the field due to turning, operator error, and variable field conditions which affect wheel slip. Constant or precise speed is dependent on operator skill, uniform field conditions and slope. Secondly, the crop canopy volume, shape or density may vary throughout the field causing non-uniform conditions. This can be caused by either incomplete plant coverage of the field area, such as missing or small plants, or natural differences in plant foliage density. As a result, these variations can lead to over-application of chemical in some areas because the maximum application rate is often selected for the entire field. Therefore, uneven application rates can result with either variable speeds or variable crop condition causing variation in spray rate and/or chemical rate.
Methods have been investigated that can monitor both the sprayer unit ground speed and the density or presence of plant material. Such methods, described in U.S. Pat. No. 4,823,268, for example, allow for changing the application rate with respect to ground speed and to plant canopy volume and shape. However, many spray systems heretofore employed utilized pressure variation to control flowrate and did not maintain a constant droplet size spectrum or a constant spray pattern as the nozzle flowrate was varied. This led to problems of spray drift (small droplets) or inadequate spray coverage (large droplets) resulting in environmental pollution and poor pest control.
In other spray systems, such as that described in U.S. Pat. No. 4,823,268, flowrate from the sprayer was controlled by adjusting the number of spray nozzles or spray nozzle manifolds which were operating. With such a method, only a few discrete levels of flowrate, rather than a continuous range of flowrate are possible. Using this method, the spray pattern from the sprayer may also be adversely affected as flow is varied.
Control of sprayer output using pressure variation at the nozzles introduces two undesirable effects in the chemical application process. First, the system response to changes in speed of the spray system vehicle can be unacceptably slow. In operation, the speed error must first occur, be sensed by the system, a control decision be made and physically implemented. Such systems can only respond to application errors rather than anticipate and prevent them. Further, the pressure-type flow control valves heretofore used for sprayer actuation were generally slow in response. With a slow response rate, use of such control systems could result in increased, rather than reduced application errors. If a ground speed variation was brief in duration, the system would not respond with an altered sprayer flow rate until the speed has returned to normal. In such cases, use of the system resulted in two occurrences of application error, rather than the single error if no control system had been used.
The second undesirable effect of pressure variation systems involves the atomization process in the spray nozzles. With conventional agricultural spray nozzles, the liquid flow rate liquid supply pressure, droplet size spectra and spray distribution pattern of the spray are interrelated. When the flow rate of liquid through the nozzle is controlled by varying the supply pressure, the droplet size spectrum and the distribution pattern of the spray cloud are also altered. The droplet size variation can be unacceptable as droplet size is often a prime design consideration in sprayer system design.
An additional disadvantage of pressure variation for flow control is the non-linear (square-root) relationship between operating pressure and resulting flowrate of liquid through a nozzle orifice. For example, to achieve a 3:1 range of flow control, a 9:1 range of pressure variation would be necessary.
Recognizing the aforesaid deficiencies and limitations of previous methods and systems it is therefore one object of the present invention to provide a variable-flow spraying system including a controller capable of varying the flowrate through a nozzle without adversely affecting the spray characteristics of the nozzle and more specifically, without distorting the droplet size spectra or distribution pattern of the nozzle.
Another object of the present invention is to provide a spraying system with a flow control device that can be electrically actuated by signals compatible with digital, micro-processor-based controllers.
Another object of the present invention is to provide a spraying device that can be compatible with and directly coupled to conventional agricultural spray nozzles.
Another object of the present invention is to provide a spraying device than can operate to produce rapid changes in nozzle flow rate when desired, utilizing a dynamic response time less than 80 ms.